In our U.S. Pat. Nos. 4,116,857 and 4,183,900 it is shown that microcrystalline MgX.sub.2 .multidot.2Al(OH).sub.3, where X is halide, prepared as a dispersion in situ within a particulate macroporous anion exchange resin is useful in selectively removing Mg.sup.++ values from brines. It is understood, of course, that the crystals may contain waters of hydration. Additional embodiments, preparations, and uses of the crystalline MgX.sub.2 .multidot.2Al(OH).sub.3 are the principal subjects of this application.
Aside from the many naturally-occurring aluminates, there are synthesized metal aluminates such as found, e.g., in U.S. Pat. Nos. 2,395,931, 2,413,184, 3,300,577 and 3,567,472.